Network defects in polyacrylamide gels

The weight conversion at the gel point (αGᵂ) was determined by the gas bubble method in two sets of polyacrylamide (PAA) gels crosslinked with N,N′-methylene-bisacrylamide (BA). αGᵂ remained practically constant when the crosslinker ratio was C = 10% (w/w) and the total concentration of comonomers in the feed ranged from CT = 5 to 10 g/100 ml. When CT was kept constant at 5 g/100 ml and C changed from 0 to 25% (w/w), αGᵂ showed a minimum at C = 7%. By comparing the experimental values of αGᵂ with those calculated for an ideal network, it was concluded that the crosslinking efficiency of BA is very low. The dependence on C of αGᵂ and other properties like the elastic phantom modulus, |ƒₚₕ*|, and the polymer volume fraction at swelling equilibrium, v₂ₛo, was explained in terms of the formation of network defects: the formation of BA sequences longer than unity and that of intramolecular cycles both have an increasing contribution at C > 7% but can be neglected at lower values of C.This work has been supported by a grant from CAICYT No. GG85-0013. The authors thank Dr A. Horta and Dr J. J. Freire for many helpful discussions